Many applications require a multiplicity of workpieces of the same type, and of the same shape, in slice form, which are obtained by cutting from a stock piece. A slice is a cylinder, the height of which is small compared with the dimensions of its base surface. A cylinder is a body bounded by two parallel, plane and congruent base surfaces and a lateral surface, the lateral surface being spanned by all parallel straight lines which intersect the edges of the two base surfaces.
Slices of right cylindrical shape are important. In a right cylinder, the straight lines of the lateral surface extend perpendicularly to the base surfaces. Right cylindrical slices with a polygonal base surface, i.e. right prisms, or circular base surface, i.e. right circular cylinders, are of particular importance.
Examples of right cylindrical slices with a polygonal base surface are photovoltaic cells (“solar cells”), the base surfaces of which are usually square or approximately octagonal.
Examples of right cylindrical slices with a circular base surface are slices of semiconductor material, which are used as substrates for the patterning of electronic, microelectronic or microelectromechanical components, or which are used as a support for the deposition of various coatings.
Semiconductor materials comprise elemental semiconductors, such as silicon or germanium, and compound semiconductors, for example gallium arsenide or silicon carbide, or composites or layered structures thereof. Examples of supports for the deposition of various coatings are slices of silicon, gallium arsenide or silicon carbide, on which for example gallium arsenide is applied in order to produce semiconductor lasers or LEDs (light-emitting diodes), or slices of aluminum, glass or ceramic, on which magnetizable layers are applied in order to produce hard-disk memories, or supports made of glass for the application of optical coatings (mirrors, filters), so-called optical flats. Further examples are slices of optically birefringent crystals such as potassium dihydrogen phosphate (KDP), lithium niobate, etc., for use in nonlinear optics (frequency doubling in lasers), slices of sapphire (Al2O3), ceramic, and many others.
Often, these slices are also referred to as wafers, particularly those made of silicon (photovoltaics, microelectronics) or gallium arsenide and silicon carbide (optoelectronics). Usually, one of the two base surfaces of the cylindrical wafers is designated as a component- or function-carrying side in relation to the base surface lying opposite it. The side designated in this way is then referred to as the front side, and the side lying opposite the front side as the back side of the wafer.
The stock pieces from which the wafers are cut are also referred to as ingots. These ingots generally have a cylindrical, usually right cylindrical, shape, the base surface of which is congruent with that of the wafers obtained from it. The main axis of inertia of the ingot with the lowest moment of inertia is referred to as the ingot axis. For right prismatic ingots with a regular polygonal base surface, or for ingots with a right circular-cylindrical shape, the ingot axis is the same as the symmetry axis of the ingot.
Semiconductor wafers as substrates for microelectronic components are usually provided in the edge region with a notch or a flat. The notch or flat marks a designated crystal direction and is applied on the ingot by milling an axial groove, the notch groove, or grinding an axial flat, the flat surface, before the cutting into wafers. After the cutting, the semiconductor wafers are usually provided with an identification code on their front or rear side by laser scribing in the immediate vicinity of the direction mark.
Cutting methods referred to as “wire sawing” have particular importance for cutting the ingots into wafers. In wire sawing, the entire ingot is cut simultaneously into a multiplicity of wafers of the same type in a device referred to as a wire saw. Wire sawing is therefore a discontinuous batch process. A device suitable for carrying out wire sawing is referred to as a wire saw.
A wire saw comprises wire, at least two cylindrical wire guide rollers, a device for holding and moving the ingot, and grinding agent. The axes of the ingot and of the wire guide rollers are arranged parallel to one another. The lateral surfaces of the wire guide rollers are provided with a multiplicity of parallel grooves, essentially equidistant from one another, which are respectively continuous and extend perpendicularly to the wire guide roller axis. The wire is guided in a spiral externally around the wire guide rollers, in such a way that it respectively comes to lie precisely once in each groove of each wire guide roller, and a wire web consisting of wire sections extending parallel to one another and perpendicularly to the wire guide roller axis is tensioned between two wire guide rollers.
The cutting process comprises movement of the wire in the longitudinal wire direction by rotation of all the wire guide rollers in the same sense with the same circumferential speed, feeding of the ingot perpendicularly onto the wire web, and the supply of grinding agent. By the relative movement of the wire with respect to the ingot, with the aid of the grinding agent the wire causes material abrasion from the ingot upon contact with the ingot and during continued feeding of the ingot. With continued feeding, the wire web thus works slowly through the ingot and produces a multiplicity of wafers of the same type simultaneously.
For most applications, wafers of exactly equal thickness are required. Since the wire is subjected to a thickness decrease due to wear during the cutting, the grooves in the wire guide rollers are usually provided with distances from one another decreasing slightly from the fresh wire side to the used wire side.
On its side facing away from the wire web at the start of cutting, the ingot is adhesively bonded to an ingot mounting beam. The cutting process is ended as soon as all the wire sections of the wire web have cut through the ingot fully, and have fully arrived in the ingot mounting beam. The cut wafers then remain suspended from the half-cut mounting beam like teeth on a comb, and are still connected along a part of their lateral surfaces by the adhesive joint to the ingot mounting beam which has been cut into. The ingot mounting beam consists of a material which is easy to cut, for example hard carbon, plastic, a mineral material or a composite of these or other materials.
By reversing the feed direction, the ingot—now cut into wafers—is moved out of the wire web and the wafers are separated by releasing the adhesive bond. The release of the adhesive bond is referred to as uncementing. The adhesives used are, for example, soluble in water, by modifying the pH, soluble in solvent or thermally soluble, so that by immersing the cut ingot into a suitable liquid or heating, all the wafers can be uncemented simultaneously, or the adhesive joints are separated successively by breaking, cutting, laser or water jet separation, and the ingot is separated wafer by wafer.
The length to which each wire section extends inside the ingot at any time of the cutting is referred to as the engagement length of the respective wire section. The greatest engagement length which occurs throughout the cutting process is referred to as the diameter of the ingot. The moment of the cutting process at which the wire comes in contact with the workpiece for the first time is referred to as cut-in. For non-rotationally symmetrical ingots, the diameter as defined above is therefore dependent on the orientation (angular position) with which the ingot is cemented onto the ingot mounting beam.
The various wire sawing methods may be distinguished according to the engagement length: in the case of cuboid ingots, which are orientated with a side surface parallel to the wire web, the engagement length is constant throughout the cutting process for all the wire sections. In the case of generally prismatic but not cuboid ingots, which are orientated with a side surface parallel to the wire web, the engagement length at cut-in is finite and generally variable in the further course of the cutting process. In the case of ingots which do not have a side surface orientated parallel to the wire web, the engagement length at cut-in is zero, then initially increases in the further course of the cutting process, and is generally variable and finite throughout the cutting process. In the case of circular-cylindrical ingots, it is zero at cut-in, then increases to a maximum, before subsequently decreasing and decreasing to zero again upon exit.
The various wire sawing processes may furthermore be subdivided into lapping and grinding, according to the mechanism of the material abrasion:
In the case of lapping, a suspension of abrasively acting hard substances is supplied to the wire. The material erosion takes place by a three-body interaction (1 ingot, 2 abrasive, 3 wire) by means of lapping. Lapping refers to breaking of the material cohesion by locally exceeding the material strength by Hertzian pressing with microcrack formation between freely mobile abrasives and the workpiece surface (brittle erosive abrasion). The suspension of the abrasives in a carrier liquid is also referred to as a slurry.
In the case of grinding, abrasively acting hard substances are fixed into the surface of the wire. The wire acts as a tool carrier, the fixed abrasives act as tools, and the material erosion takes place by a two-body interaction (1 ingot, 2 abrasive) by means of grinding. Grinding refers to breaking of the material cohesion by cut-in of a spatially invariantly oriented cutting edge and removal of cuttings by carving through the workpiece surface parallel to the workpiece surface.
A chip is intended to mean a fragment of the workpiece released from the workpiece by the cutting action. The abrasives have the shape of irregular polyhedra (many-sided bodies). The abrasives are also referred to as grains. A cutting edge refers to the edge of a face of the polyhedron, oriented in the movement direction of the abrasive and coming in contact with the workpiece, at which edge the workpiece material is cut through and a chip thus released. The cutting angle is intended to mean the angle at which the face of the grain with the cutting edge engaging with the workpiece is placed with respect to the surface of the workpiece.
In the case of lapping, owing to its free movement in the slurry, each grain has cutting edges and cutting angles which vary over time. In the case of grinding, although each individual grain has a respectively time-invariant cutting edge with a time-invariant cutting angle owing to its fixed connection to the tool carrier (wire), ignoring wear of the grain, for example due to splintering during the grinding process, so that new cutting edges may be formed, all of the cutting edges engaging during the grinding and cutting angles of all the grains are random cutting faces and cutting angles. Lapping and grinding are therefore referred to as cutting methods with geometrically undetermined cutting edges.
Lastly, the various wire sawing methods may be distinguished according to the nature of the wire movement, in a saw with unidirectional wire movement or a saw with continual direction reversal of the wire movement.
In the case of unidirectional sawing, the wire is wound throughout the cutting process in precisely one longitudinal wire direction from a feed spool to a take-up spool. In the case of sawing with continual direction reversal of the wire movement, the direction of the longitudinal wire movement is continually reversed. Within the group of sawing methods with continual direction reversal of the wire movement, the reciprocating step method is particularly important.
According to the reciprocating step method, the cutting consists of a sequence of so-called reciprocating steps or “pilgrim steps”. A reciprocating step comprises precisely movement of the wire through a first length in a first longitudinal wire direction and subsequent movement of the wire through a second length in a second direction, precisely the opposite of the first direction, the second length being selected to be shorter than the first length. During a reciprocating step, a wire length corresponding in total to the sum of the two lengths thus passes through the workpiece, while the wire section coming in cutting engagement with the workpiece is moved forward from the feed spool to the take-up spool in total only by an amount corresponding to the difference between the two lengths. In the reciprocating step method, the wire is thus used repeatedly by the factor given by the ratio of the sum to the difference of the two lengths. The difference between the two lengths is also referred to as the “net movement” of the wire over a full reciprocating step.
The wire contains for example plastic, carbon fibers, or metal alloys with one or more strands (cable). Monofilament hardened steel wire (piano wire) is particularly important. The steel wire used during lapping is coated with a nonferrous metal alloy, usually with a layer thickness of less than one micrometer, which derives as a lubricant from the wire drawing process and counteracts corrosion. The steel wire used during grinding is coated with a layer of synthetic resin or nickel, which acts as a binder for the fixed abrasives. In the case of grinding wire, the abrasives may also be fixed by a form fit, for example by rolling (pressing) the abrasive into the surface of the steel wire.
The abrasives used in the case of lapping comprise for example silicon carbide, boron carbide, boron nitride, silicon nitride, zirconium oxide, silicon dioxide, aluminum oxide, chromium oxide, titanium nitride, tungsten carbide, titanium carbide, vanadium carbide, diamond, sapphire and mixtures thereof. Silicon carbide is particularly important for lapping and diamond is particularly important for grinding.
The carrier liquid of the slurry comprises, for example, oil or glycol.
Slurry lapping and a device suitable therefor, for cutting semiconductor wafers, are described for example in EP 0 789 091 A2. Diamond wire grinding and a device suitable therefor, for cutting semiconductor wafers, are described for example in WO 2013/041140 A1.
An ingot has a start, an end and a middle. The ingot start refers to the axial region of the ingot close to the end surface of the ingot during wire sawing with a unidirectional longitudinal wire movement, in the vicinity of which end surface the wire engages with the ingot for the first time during its movement in the longitudinal wire direction; in the case of wire sawing with continual direction reversal of the longitudinal wire movement (reciprocating method), correspondingly the ingot start refers to the axial region of the ingot in the vicinity of the end surface, in the vicinity of which the wire engages with the ingot for the first time during its net movement over a full reciprocating step. The ingot end refers to the axial region in the vicinity of the end surface of the ingot on the opposite side from the ingot start, and the ingot middle refers to the region between the ingot start and the ingot end.
Correspondingly, the wire web also has a start, a middle and an end. The wire web start refers to the part whose wire sections cut through the ingot section at the ingot start; the wire web end refers to the part whose wire sections cut through the ingot end, and the wire web middle refers to the part whose wire sections cut through the ingot middle. In the course of its longitudinal wire movement in the case of unidirectional sawing, or in the course of its net movement when sawing with the reciprocating step method, the wire enters the wire web at the wire web start and emerges from the wire web at the wire web end.
Each kerf formed in the ingot by chip formation has a wire entry side and a wire exit side. The wire entry side refers to the side of the ingot in the longitudinal wire direction on which the wire enters the kerf during its longitudinal wire movement (unidirectional cut) or during its net wire movement (cutting with the reciprocating step method); the wire exit side refers to the side on which the wire emerges from the cutting kerf.
During the cutting process, the wire sections are subjected to a transverse deflection in the feed direction. This is also referred to as wire bending. The wire bending results from the prestress of the wire in the longitudinal wire direction and the elasticity of the wire, in response to a transverse wire force acting in the transverse wire direction in the feed direction. The transverse wire force in the feed direction is an essential part of the cutting process. Without this transverse wire force, the grain cannot penetrate into the workpiece, and no material erosion takes place. The transverse wire force in the feed direction is determined by the ratio of the material removal rate and the speed of the longitudinal wire movement.
The material removal rate refers to the volume of chips generated per unit time, which are released from the workpiece by the cutting process. For wire bending which is small compared with the free length of the individual wire sections between their bearing points on the two wire guide rollers tensioning the wire web, the wire bending is proportional to the ratio of the material removal rate and the longitudinal wire speed (linear range, Hooke's law). In this sense, only small wire bending occurs during the wire sawing.
Since the wire exhibits no wire bending before the cut-in, and finite wire bending during the cutting of the workpiece, at least in the cut-in region there is always a region in which the wire bending varies.
For example, the wire bending increases with an increasing engagement length, increasing speed of the feed of the workpiece onto the wire web (increasing material removal rate) and decreasing speed of longitudinal wire movement, and it decreases with decreasing engagement length, decreasing material removal rate and an increasing speed of the longitudinal wire movement.
Cut-in beam refers to a body which is fastened on the ingot at the cut-in position, so that during the cutting process the wire web initially engages with the cut-in beam and only engages with the ingot after at least partially cutting through the cut-in beam. The purpose of cut-in beams, in the case of cutting workpieces with an engagement length that varies with cut depth, is to minimize the variation of the engagement length at least in the cut-in region.
Known cut-in beams are distinguished in that the extent of the cut-in beams in the ingot advance direction is small compared with the ingot diameter.
JP2007-301688 A2 describes a wire sawing method in which a cut-in beam is used.
Many of said wafers cut from ingots by means of wire sawing are intended for particularly demanding applications which require a particularly high degree of planarity and parallelism of the front and back sides of the wafers.
Among persons skilled in the art of wire sawing, it is known that the sidewalls of the kerfs formed by the wire sections of the wire web in the ingot are usually not exactly flat. In particular, it is known that the wire sections initially cut into the ingot upon first contact with the lateral surface of the ingot (cut-in process) at axial ingot positions which differ slightly from those at which they continue with further cutting in the course of the further ingot feed onto the wire web. This leads to wafers which have relatively pronounced deviations from a desired perfect plane-parallelism of their front and back sides, particularly in the cut-in region. Such a deviation, in the same direction, of the front and back sides from planarity in the cut-in region may be referred to as a “cut-in wave”.
Known cut-in beams are unsuitable for solving the problem of long-wavelength deviations of the front and back sides of the wafers obtained by cutting the ingot from the desired plane-parallelism with one another in the particularly critical cut-in region.
Irregular cut-in can lead to planarity deviations, not in the same direction, of the respective front and back sides of the cut wafers. In particular, under certain circumstances the wafers are thinner in the cut-in region than at other positions. This form of nonplanarity in the cut-in region of a wafer may be referred to as a “cut-in wedge”.